Field of the Invention
The present invention relates to an image pickup lens, and in particular, to an image pickup lens of an interchangeable lens camera.
Description of the Related Art
As an interchangeable lens camera, a mirrorless single-lens camera has been known. In the mirrorless single-lens camera, there is no bending mirror as in a single-lens reflex camera. Therefore, it is possible to make the mirrorless single-lens camera to be a camera that is more compact as compared to the single-lens reflex camera. In recent years, the compactness of the mirrorless single-lens cameras has been gaining acceptance by users, and the market for such cameras has been expanding.
Even the mirrorless single-lens camera has been equipped with an AU (autofocus) function similarly as the single-lens reflex camera. As an AF type, a phase-difference AF and a contrast AF are available. In the single-lens reflex cameras, the phase-difference AF has been predominant.
On the other hand, among the mirrorless single-lens cameras, there are cameras in which the phase-difference AF cannot be used. In such mirrorless single-lens cameras, the contrast AF is to be used. In the contrast AF, the focusing is to be carried out by finding a location by scanning a focusing lens unit, where the contrast becomes the maximum.
A problem that arises in a case of using the contrast AF is a weight of the focusing lens unit. Here, an amount of movement of the focusing lens unit which is necessary till a focused state achieved is let to be a predetermined amount of movement. In a case of the phase-difference AF, it is possible to calculate the predetermined amount of movement at one time by using information from an AF sensor. Accordingly, it is possible to move the focusing lens unit in accordance with the predetermined amount of movement that has been calculated.
Whereas, in a case of the contrast AF, information obtained from the AF sensor is only a contrast value at that instant. In other words, the predetermined amount of movement cannot be determined at one time. Therefore, in the contrast AF, the contrast is calculated by moving the focusing lens unit, and the contrast calculated is compared with the contrast before moving. In such manner, while reading out a change in the contrast, the focusing operation is carried out by finding a location where the contrast becomes the maximum.
Here, for detecting the maximum value of contrast, it is to be made sure that the contrast after moving becomes smaller than the contrast before moving. Accordingly, in the contrast AF, it is necessary to move the focusing lens unit to a position farther than a position at which, the contrast became the maximum.
Consequently, when the predetermined amount of movement is compared for the contrast AF and the phase-difference AF, the predetermined amount of movement is overwhelmingly larger for the former. As described above, in an optical system in which, the contrast AF is used, light-weighting of the focusing lens unit becomes a vital point.
As an image pickup lens in which, the contrast AF is to be used, optical systems disclosed in Japanese Patent Application Laid-open Publication Nos. 2012-255842 and 2014-35458 are available.